1. Field of the Invention
The present invention relates to a machining method and a machining apparatus in which a table mounted with a work and a main shaft holding a tool are moved relatively to each other in X-, Y- and Z-directions perpendicular to one another for machining the work with the tool.
2. Description of the Related Art
FIG. 8 is a configuration diagram of a printed wiring board machining apparatus used conventionally. In FIG. 8, a printed wiring board 1 is fixed to the top of a table 2 with an underplate 9. The table 2 can be moved desirably in the front/rear (X) direction by an X-axis drive unit 3. A spindle 5 holding a drill 4 can be moved desirably in the up/down (Z) direction by a Z-axis drive unit 6. The Z-axis drive unit 6 can be moved desirably in the left/right (Y) direction by a Y-axis drive unit 7. Each of the X-, Y- and Z-axis drive units 3, 7 and 6 has a not-shown position detector, so that the drive unit can be positioned accurately in a specified position (coordinates) by feedback control.
At the time of machining, the X-axis drive unit 3 and the Y-axis drive unit 7 are operated so that the axis of the drill 4 is positioned on the axis (perpendicular to the printed wiring board 1) of a drilling position 8. After that, the spindle 5 is moved down to a specified height by the Z-axis drive unit 6, and the printed board 1 is drilled at an aimed position. When drilling finishes, the spindle 5 is lifted up, and the drill 4 is brought back to its standby position. Then, the aforementioned operation is repeated till machining is terminated. Therefore, the machining time is calculated as the sum of the X/Y-direction movement time and the Z-direction movement time.
The standby position of the drill 4 is defined as a position where the forward end of the drill 4 is at a predetermined distance from the surface of the printed wiring board 1 in order to discharge swarf surely and prevent the drill 4 from interfering with the printed wiring board 1 when the drill 4 moves horizontally. The distance from the forward end of the drill 4 to the surface of the printed wiring board 1 in the standby position is called an air-cut distance La. The value of the air-cut distance La is often determined based on experience. This value determined based on experience corresponds to the aforementioned predetermined distance.
FIG. 9 is a diagram showing the relationship between a positioning command and a real position (response) of the X-axis drive unit 3 with the abscissa designating the temporal axis. FIG. 10 is a diagram showing the relationship between a poisoning command and a real position (response) of the Z-axis drive unit 6 likewise.
As shown in FIG. 9, the position response of the X-axis drive unit 3 delays with respect to the X-axis positioning command. In addition, at the time of stopping, the position response oscillates (overshoots or undershoots) with respect to an aimed position, and the X-axis drive unit 3 approaches the aimed position gradually. The time required since the X-axis positioning command reached the aimed position and till the X-axis drive unit 3 is stabilized within a predetermined allowable position error range is referred to as a stabilization time Ts. On the other hand, the operation of the Y-axis drive unit 7 is similar to that of the X-axis drive unit 3.
On the other hand, as shown in FIG. 10, due to a lighter load on the Z-axis drive unit 6 than that on the X-axis drive unit 3, the delay (position deviation) of the position response of the Z-axis drive unit 6 with respect to the Z-axis poisoning command is smaller than that of the X-axis drive unit 3. In addition, the acceleration time of the Z-axis drive unit 6 is shorter. Thus, the Z-axis drive unit 6 lowers down substantially at a velocity specified as machining conditions, and rises up at a maximum velocity as soon as drilling finishes.
FIG. 11 is an explanatory diagram of an invented machining procedure by which the machining speed can be improved. In order to shorten the machining time and improve the machining accuracy, the drill 4 may be made to cut into the printed board 1 as soon as the axis of the drill 4 is positioned on the axis (vertically above) of the drilling position 8, that is, as soon as the stabilization time Ts has passed, as shown in FIG. 11.
To this end, as disclosed in Japanese Patent Laid-Open No. 2002-166396, the present inventor proposed an invention in which the timing to position the axis of the drill 4 at a machining position is calculated from the movement distance, the velocity and the acceleration in the case where the table 2 and the drill 4 move in the X and Y directions, and the forward end of the drill 4 is made to reach the top of the printed wiring board synchronously with the end point of the stabilization time Ts.
According to this invention, when it is assumed that the velocity of the Z-axis drive unit 6 is fixed and there is no delay in position response of the Z-axis drive unit 6 with respect to a command given thereto, the movement time Ta for the drill 4 to reach the top of the board can be obtained from Equation 1 using the lowering velocity Vz and the air-cut distance La of the Z-axis drive unit 6.Ta=La/Vz  Equation 1
The lowering motion of the Z-axis drive unit 6 is started:
at the same time as the command-reach time of the X-axis when Ts=Ta;
after the time (Ts−Ta) has passed from the command-reach time of the X-axis when Ts>Ta; and
the time (Ta−Ts) prior to the command-reach time of the X-axis when Ta>Ts.
Incidentally, for moving to the next machining position, the X-axis drive unit 3 and the Y-axis drive unit 7 are operated simultaneously. Therefore, the timing at which Z-axis operation is started is determined in accordance with the slower drive unit which will reach the next machining position slower.
However, the timing obtained in the aforementioned related art is a theoretical timing. A real machining apparatus has a variation in properties. In addition, even in the same machining apparatus, the properties may change, for example, owing to a movement direction, a movement start position, or the like. The properties may also change owing to the maintenance conditions or the environment where the apparatus is installed. Therefore, in order to keep high machining accuracy, it may be necessary to measure the drilling position accuracy in a really drilled printed wiring board. When such measurement is carried out, improvement in machining productivity cannot be expected owing to the measuring step.